[Bug 86478] [HSW Bisected] *ERROR* uncleared pch fifo underrun on pch transcoder A & *ERROR* uncleared fifo underrun on pipe B while plugging in VGA

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=86478

Li Xu <li.l.xu at intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[HSW Bisected] *ERROR*      |[HSW Bisected] *ERROR*
                   |uncleared pch fifo underrun |uncleared pch fifo underrun
                   |on pch transcoder A         |on pch transcoder A &
                   |                            |*ERROR* uncleared fifo
                   |                            |underrun on pipe B while
                   |                            |plugging in VGA

--- Comment #1 from Li Xu <li.l.xu at intel.com> ---
Environment:
--------------------------
Platform: Haswell
Kernel: (drm-intel-next-queued) latest
77c1aa84de0096792de673aa1c64c36b38553cf5

dmesg log is same as bug Bug 86233:
https://bugs.freedesktop.org/attachment.cgi?id=109444

Bug detailed description:
-----------------------------
When boot up ,and then plug in VGA ,error messages show as follows:

*ERROR* uncleared pch fifo underrun on pch transcoder A
*ERROR* PCH transcoder A FIFO underrun
*ERROR* uncleared fifo underrun on pipe B
*ERROR* CPU pipe B FIFO underrun

Steps:
---------------------------
Boot up
plug in VGA

Bisect Result
---------------------------
The _culprit_ commit introduced the following two error lines.
*ERROR* uncleared pch fifo underrun on pch transcoder A
*ERROR* uncleared fifo underrun on pipe B

29c6b0c5ee33d54ada586040eca2a258f0ce0650 is the first bad commit
commit 29c6b0c5ee33d54ada586040eca2a258f0ce0650
Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
Date:   Fri May 16 19:40:24 2014 +0300

    drm/i915: Simplify the uncleared FIFO underrun detection

    Checking whether the error interrupt was enabled or not isn't really
    necessary when we check for uncleared FIFO underruns. If it was enabled
    we'll race with the interrupt handler a bit, but that seems OK as we
    still claim the interrupt.

    Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
    Reviewed-by: Thomas Wood <thomas.wood at intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

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